In the case of a bidirectional communication where data are transmitted and received at the same time through a transmission bus, it is important to determine whether or not data exist on the transmission in order to prevent a collision caused by transmission from the opposite side during transmission.
Furthermore, in order to prevent a receiver from recognizing noise as data, it is necessary to discriminate signals from noise. Therefore, when the bidirectional communication is performed on the same transmission line, a squelch detection circuit to detect whether data exist or not is additionally required in addition to data transmission and reception.
The squelch detection circuit serves to cause a signal received by a communication device or the like to be heard in a state where noise is reduced.
In general, the squelch detection circuit recognizes an input signal as noise when the magnitude of the input signal is smaller than a designated magnitude, and recognizes an input signal as a signal when the magnitude of the input signal is larger than the designated magnitude.
The squelch detection circuit described in the specification of Universal Serial Bus 2.0 (hereinafter, referred to as USB 2.0) must indicate a squelch state in which no data are transmitted, when a voltage difference between differential signals is smaller than 100 mV.
Meanwhile, in a serial interface, a transmitter and a receiver do not use the same clock signal, but the receiver uses a clock signal restored by using data transmitted from the transmitter. Therefore, the receiver uses an elastic buffer.
Accordingly, when data are transmitted in the unit of packet as in USB 2.0, the elastic buffer must be reset in a state where no data are transmitted.
Furthermore, the voltage of a differential-mode signal received by the receiver must be set in a range to operate an input terminal of a receiver amplifier. However, since a common-mode signal voltage does not have a desired value in many cases, a level shifter may be used to give an offset to a reset terminal.
However, the conventional squelch detection circuit does not provide a circuit which is required in the USB 2.0 specification and not only detects a squelch state, but also resets the elastic buffer.